The present invention relates to an implant system, specifically a dental implant system for a partially or fully edentulous patient which comprises a fixture for implantation into the bone tissue of the maxilla or mandible of the patient and a superstructure for mounting to the fixture which in turn supports or presents a dental restoration comprising one or more artificial replacement teeth. As an example, the superstructure may be an abutment which is adapted in use to be connected to the implanted fixture to bridge the soft tissue layer (gingiva) overlying the maxilla or mandible for a restoration structure such as a crown or bridge to be secured thereto.
In a typical dental implant system the superstructure is screw retained to the fixture. To this end, the fixture has an internally threaded socket which opens in a rear end of the fixture. The internal threads may be present throughout the socket or alternatively just in a is section of the socket. The superstructure is then either (i) integrally formed with an externally threaded projection for screwing into the internally threaded socket to secure the superstructure to the fixture, or (ii) comprises a sleeve part having a lumen in which a transverse bearing surface is provided, and a screw part for passage through the lumen of the sleeve into the internally threaded socket, the screw having an enlarged head which is adapted in use to bear against the transverse bearing surface in the lumen of the sleeve thereby ensuring that the superstructure is secured to the fixture on screwing of the screw into the internally threaded socket.
In other dental implant systems the superstructure is secured to the fixture through non-screw thread connections, e.g. by cementing.
Whatever the form of securement of the superstructure to the fixture, the superstructure transmits a load to the fixture in the implantation direction through mutually interfacing surfaces.
Fixation of a fixture of a dental implant system in the bone tissue of the maxilla or mandible primarily relies upon mechanical interlocking of bone tissue to the outer surface of the fixture. Substantial interlocking between the outer surface of a fixture and the bone tissue of the maxilla or mandible occurs when the outer surface is roughened, for example by providing the fixture outer surface with screw threads, recesses, holes etc. (macroroughening), forming pits in the fixture outer surface by blasting, plasma spraying, etching, hydroxyapatite coating, the provision of beads on the outer surface etc. (microroughening) or machining the fixture outer surface. Interlocking between the outer surface of a fixture and bone tissue of the maxilla or mandible into which the fixture is implanted enables the fixture to greater withstand shear stresses. One can consider this to be due to bone tissue growing into the recesses in the fixture outer surface created by the roughening.
A macroroughened fixture outer surface produces more effective interlocking than a microroughened fixture outer surface which in turn produces more effective interlocking than a machined fixture outer surface. All, however, produce more effective interlocking than fixtures having an outer surface which is smooth, e.g. a polished outer surface. More than one form of surface roughening can be applied to a fixture to promote interlocking. For example, it is known to provide a fixture with both a macro- and a microroughened outer surface.
In 1892 it was suggested by J. Wolff (Das Gesetz der Transformation der Knochen. Berlin: A. Hirschald, 1892) that bone tissue remodels itself in response to the mechanical loading history of the bone and in accordance with mathematical laws. This has since proved to be correct and is therefore now known as Wolff""s law. Some consequences of Wolff""s law are that
1. If the stresses or strains to which bone tissue is exposed are lowered a net loss of bone tissue (bone resorption) will result until a new steady state is obtained where the stresses or strains are normal.
2. If the stresses or strains to which bone tissue is exposed are normal then no net change to the bone tissue will result.
3. If the stresses or strains to which bone tissue is exposed are increased within physiological limits bone tissue build up will result until a new steady state is obtained where the stresses or strains are normal.
4. If the stresses or strains to which bone tissue is exposed are extremely high then bone resorption will result.
Wolff""s law thus indicates the need for stress conditions in the bone tissue adjacent an implant fixture which are favourable for the maintenance of a steady state in the bone tissue. Otherwise, marginal bone resorption will result which will cause destabilisation of the osseointegration of the fixture with the bone tissue and a reduction in the aesthetic appeal of the dental implant system when implanted, an important commercial consideration.
With this in mind, the Applicant""s have identified an interdependence between the stress conditions in the bone tissue adjacent an implant fixture and the position of the interfacing surfaces through which a fixture is loaded in the implantation direction by a superstructure vis-à-vis the level on the fixture outer surface where interlocking starts. The present invention proposes to provide a dental implant system which takes account of this interdependence to improve the stress conditions in the bone tissue adjacent the outer surface of an implanted fixture.
According to the present invention there is provided a dental implant system comprising:
a fixture for implantation in bone tissue of the maxilla or mandible by displacement thereof in a forward direction, the fixture having:
a forward end,
a rearward end, and
an outer surface which extends between the forward and rearward ends and which forwardly of a predetermined position on the outer surface is adapted for interlocking with bone tissue of the maxilla or mandible; and
a superstructure for mounting on the fixture, the superstructure having:
a forward end for interfacing with the rearward end of the fixture, the forward end of the superstructure and the rearward end of the fixture presenting interfacing surfaces for interfacing with one another;
characterised in that the rearward end of the fixture and the forward end of the superstructure are so constructed and dimensioned that a predetermined interfacing surface of the superstructure is able to interface in the forward direction with a predetermined interfacing surface of the fixture which is at a level which is disposed forwardly of the predetermined position on the outer surface.
The result of this arrangement is that when the fixture is implanted the superstructure applies a load to the fixture in the forward direction through interfacing surfaces at a level which is disposed forwardly of the predetermined position on the fixture outer surface which is the attachment level at which bone tissue starts to interlock with the fixture when implanted. Finite element analysis indicates that loading of a fixture in the forward direction forwardly of the attachment level results in an improved stress distribution in the adjacent bone tissue of the maxilla or mandible. For example, application of a forwardly directed load through interfacing surfaces forward of the attachment level shifts the peak interfacial shear stress forwardly where the risk of triggering off stress induced bone resorption is less. In addition, the longitudinal compressive stresses (relative to the maxilla or mandible ridge) in the bone tissue and the stresses caused by the horizontal loads or bending moments on the fixture are less as one moves more forwardly into the bone tissue and therefore the peak interfacial shear stress generated in the bone tissue can be more easily accommodated there. The peak interfacial shear stress value is also generally reduced in value.
In an embodiment of the invention the predetermined interfacing surfaces of the fixture and the superstructure are the only interfacing surfaces of the fixture and superstructure which are adapted to interface with one another in the forward direction. Thus, all of the forwardly directed load received by the fixture from the superstructure is at a level below the attachment level. Finite element analysis indicates that applying all of the forwardly directed load to the fixture forwardly of the attachment level gives the greatest improvement in the stress distribution in the adjacent bone tissue of the maxilla or mandible.
The benefit of the invention is still attained, though, where the fixture is loaded in the forward direction through interfacing surfaces forward of the attachment level and interfacing surfaces at, or rearward of, the attachment level. Accordingly, in another embodiment of the invention the predetermined interfacing surfaces of the fixture and superstructure are predetermined forward interfacing surfaces and the forward end of the superstructure and the rearward end of the fixture are so constructed and dimensioned that the forward end of the superstructure presents a predetermined rearward interfacing surface for interfacing in the forward direction with a predetermined rearward interfacing surface presented by the rearward end of the fixture at a level disposed at, or rearwardly of, the predetermined position on the fixture outer surface. In this case it is preferable that a major part of the collective forwardly directed loading applied to the fixture be at a level forward of the predetermined position on the fixture outer surface or attachment level. However, provided a forwardly directed load is applied to the fixture forwardly of the attachment level, whether this be the major part or the minor part of a forwardly directed load distributed on both sides of a level coinciding with the attachment level, an improved stress distribution will result in the bone tissue adjacent the fixture as compared to the case where all of the forwardly directed load is applied at or rearward of the attachment level, as in the prior art as will be illustrated hereinafter.
An improved stress condition in the bone tissue adjacent the fixture will be obtained even if the predetermined interfacing surface of the fixture is disposed only marginally forwardly of the predetermined position on the fixture outer surface, e.g. at a level which is approximately 0.1-1 mm forward of the predetermined position on the fixture outer surface. A greater improvement will be forthcoming, however, if the predetermined interfacing surface of the fixture is disposed well forward of the predetermined position on the fixture outer surface. Thus, preferably the fixture and superstructure are adapted in use to interface at a level which is greater than 1 mm forward of the predetermined position on the fixture outer surface, more preferably greater than 3 mm forward of the predetermined position on the fixture outer surface and even more preferably greater than 5 mm forward of the predetermined position on the fixture outer surface, e.g. approximately 10 mm forward of the predetermined position on the fixture outer surface.
In embodiments of the invention hereinafter to be described a female recess is provided in the rearward end of the fixture having a boundary wall which extends from an opening in the rearward end to a level forwardly of the predetermined position on the fixture outer surface, the predetermined interfacing surface of the fixture is a predetermined surface of the boundary wall of the female recess which is disposed forwardly of the predetermined position on the fixture outer surface, and the predetermined interfacing surface of the superstructure is a predetermined surface of a boundary wall of a male projection provided at the forward end of the superstructure.
In an embodiment of the invention the predetermined surfaces of the boundary walls of the female recess and male projection have a generally conical profile with flank surfaces which converge in the forward direction at a common angle or a substantially common angle.
In an embodiment of the invention hereinafter to be described the predetermined surfaces of the boundary walls of the male projection and female recess are presented by predetermined forward sections of the respective boundary walls and the boundary walls of the male projection and female recess each have rearward section s which extend rearwardly from the predetermined forward section and which are so constructed and dimensioned that they are spaced apart when the predetermined forward sections interface. To this end, the rearward section of the boundary wall of the female recess may be of a conical profile with flank surfaces which converge in the forward direction at the common angle or substantially the common angle and the rearward section of the male projection may be of a conical profile with flank surfaces which converge in the forward direction at an angle which is more acute relative to the forward direction than the common angle.
In an embodiment of the invention hereinafter to be described the rearward section of the boundary wall of the female recess extends rearwardly to the rearward end of the fixture.
In another embodiment of the invention hereinafter to be described the predetermined surfaces of the boundary walls of the female recess and male projection are presented by predetermined forward sections of the respective boundary walls, the predetermined rearward interfacing surface of the fixture is presented by a predetermined rearward section of the boundary wall of the female recess disposed at, or rearwardly of, the predetermined position on the fixture outer surface and the predetermined rearward interfacing surface of the superstructure is presented by a predetermined rearward section of the boundary wall of the male projection. By making the extent of the predetermined forward section of the boundary wall of the female recess in the forward direction greater than the extent of the predetermined rearward section of the boundary wall of the female recess in the forward direction the major part of the forwardly directed loading applied to the fixture by the superstructure will be applied forward of the predetermined position on the fixture outer surface.
In an embodiment of the invention the predetermined forward and rearward sections of the boundary walls of the female recess and male projection are contiguous.
In an embodiment of the invention the predetermined rearward sections of the boundary walls of the female recess and male projection have a generally conical profile with flank surfaces which converge in the forward direction at the common angle or substantially the common angle.
In an alternative embodiment of the invention hereinafter to be described the predetermined surface of the boundary wall of the female recess is presented by a transverse section of the boundary wall. The transverse section of the boundary wall of the female recess may be a transverse base of the female recess. For instance, the female recess may comprise a rearward conical portion which opens in the rearward end of the fixture and a polygonal forward section which communicates with the rearward conical portion at the rearward end thereof and which presents the transverse section at the forward end thereof. An internally threaded bore could extend forwardly into the fixture from the transverse section without adversely affecting the benefit secured by the invention.
The superstructure of the system of the invention may take the form of an abutment for bridging the soft tissue layer overlying the maxilla or mandible.
In an embodiment of the invention at least a section of the outer surface of the fixture is roughened for interlocking with the bone tissue, the at least a section of the outer surface having a rearward edge and a forward edge, and the predetermined position on the outer surface is defined by the rearward edge or a position intermediate the rearward and forward edges. The latter example would be the case where the rearward edge projects from the maxilla or mandible on implantation of the fixture. The roughening may be achieved by macroroughening, e.g. by the provision of screw threads, recesses, holes etc., or by microroughening through blasting, plasma spraying, etching, hydroxyapatite coating, the provision of beads on the outer surface etc. or a mixture of both. The roughening can also be achieved by machining.
The more rearward the attachment level is disposed on the fixture outer surface the more bone tissue is involved in the load carrying. Finite element analysis also indicates that the more rearward the attachment level is disposed on the fixture outer surface the less the peak interfacial shear stress value. A rearwardly disposed attachment level is thus preferable and accordingly in an embodiment of the invention at least a forward section of the fixture which presents the forward end is adapted in use to be inserted into bone tissue for implantation of the fixture, the at least a forward section has a rearward edge and the predetermined position on the fixture outer surface is disposed at the rearward edge of the at least a forward section whereby the predetermined position on the fixture outer surface registers or substantially registers with the outer surface of the maxilla or mandible when the fixture is implanted.
Preferably, the fixture outer surface is adapted to interlock with the bone tissue between the predetermined position on the fixture outer surface and the forward end of the fixture. The predetermined position on the fixture outer surface may coincide with the rearward end of the fixture.
In an embodiment of the invention hereinafter to be described the predetermined interfacing surfaces of the fixture and superstructure are unthreaded surfaces.
In an embodiment of the invention the predetermined interfacing surfaces of the fixture and superstructure interface with one another through direct contact.
In an embodiment of the invention the fixture is an endosteal fixture of root form. By xe2x80x9croot formxe2x80x9d is meant that the fixture has a generally cylindrical shaft which in use of the fixture is implanted in the bone tissue of the maxilla or mandible. The outer surface of the shaft is roughened for interlocking with the bone tissue, e.g. the shaft may be in the form of a screw. The term xe2x80x9croot formxe2x80x9d is used to distinguish the fixture from other endosteal fixtures, e.g. the blade form. The various forms of endosteal fixture are shown in Biomaterials Science: An Introduction to Materials in Medicine, Ratner, Buddy D. et al, Chapter 7.4, Academic Press, 1996.
The present invention further provides a fixture for use in a system according to the invention and also a superstructure for use in a system according to the invention.
The invention yet further provides a method of installing a dental restoration in the oral cavity of a patient comprising the steps of providing a fixture having an outer surface at least a section of which is adapted for interlocking with bone tissue of the maxilla or mandible, providing a superstructure on which the dental restoration is formed or mountable, implanting the fixture in the maxilla or mandible of the patient in a forward direction such that at least a forward part of the at least a section of the outer surface thereof is disposed adjacent the bone tissue of the maxilla or mandible, and mounting the superstructure on the fixture such that the superstructure interfaces with the fixture in the forward direction on a surface of the fixture positioned at a level forward of the position at which interlocking between the fixture outer surface and bone tissue commences.
By way of example, embodiments of the present invention will now be described with reference to the accompanying Figures of drawings in which: